In casing structures known in the art (DE Pat. No. 2,135,577) the casing wall sections are interconnected after being introduced into a trench so that the opposite casing walls each are unitary pieces. Between the opposite casing walls which are braced against on another pipes or tubes etc. may be lowered into the trench. Soil is removed in front of the casing and dumped into the trench behind the casing. After a sufficient amount of soil is dug out in front of the casing, the complete casing is advanced by the hydraulic operating pistons and cylinders which act on the face plates in the rear of the casing. After advancement of the casing, a new operating cycle may begin. During the hydraulic advancement, the soil deposited behind the casing is, at the same time, compacted.
It has been found that elaborate equipment is required to prevent the downward movement of the front portion of such casings during operation. Such downward movement is caused by the weight of the casing but also by the resistance experienced at the lower front edge of the casing during its advancement. This generates a torque directing the front part of the casing downwardly since the advancement forces are applied by the rear face plates at a higher level and the resistance forces are generated mainly at the lower end of the casing as at the lower front edge. These forces together produce a torque which forces the front part of the casing downwardly. Furthermore, if the forces required for advancement of the casing through the ground are larger than the ground resistance of the material filled in behind the casing and serving as abutment as it might occur at greater depths and large cutting resistances, then the ground filled-in behind the casing will flow, under high pressure, upwardly along a certain sliding slope depending on the material and, in the process, causes uplifting of the rear end of the casing along with the upwardly flowing soil. Together, all the forces and phenomena cooperate to tilt the casing downwardly in the front and upwardly in the rear.
Counter measures have been developed which include stabilizer foils cutting into the ground and support gears having wheels rolling either on the surface at the sides of the trench or at the bottom of the trench. Stabilizer foils however can be used only if the soil is suitable to be cut, they cannot be utilized when the soil is rocky or includes boulders. Furthermore, they are not very effective. Support gears on the other hand require a relatively large amount of space which is objectionable especially since they need to be placed in the front part of the casing where the space is needed for the digging operation. They also require a soil which provides sufficient support which is not always available. Often support gears cannot be used simply because the space is not available as such casings are generally utilized when little space is available. Sometimes problems arise also by intersecting utility lines which may require installation of auxiliary support means in order to utilize the rear face plates to generate the hydraulic advancing forces for the casing.
It is therefore desirable to provide an arrangement in which during advancement of the casing no torque or forces are generated which cause downward tilting of the front of the casing and which permits advancing of the casing even if the hydraulic advance cylinders acting on the rear face plates cannot or not fully be utilized.